Data presented to the Emission Control Technology Division of the U.S. Environmental Protection Agency indicates some problems from using MMT at 1/32 gram Mn and 1/16 gram Mn levels with regard to exhaust gas hydrocarbon and/or carbon monoxide emissions and with regard to effect of MMT on an oxygen sensor used in three-way catalyst systems.
Three-way catalyst systems are designed to operate with exhaust gas from the engine that results from operation at close to the stoichiometric or chemically correct air/fuel ratio. With such a feedgas, the catalyst simultaneously controls HC, CO, and NOx emissions. An oxygen sensor is used with these systems as the sensor in a feedback control system which maintains the air/fuel ratio at the stoichiometric point by controlling the fuel metering system. Any deterioration of the oxygen sensor's ability to provide the appropriate signal for the feedback control system may result in a corresponding deterioration in emission control capability.
Improved oxygen sensors which are less sensitive to MMT have become available. Further, the sensitivity problem is of less concern where the three-way catalyst system employs air injection into the exhaust gas downstream of the three-way catalyst and ahead of an oxidation catalyst.
Also, studies of automotive engines run on MMT-containing gasoline with 1/32 g Mn/gal and 1/16 g Mn/gal indicate an increase in combustion chamber deposits and a resultant increase in engine-out HC emissions.
It has been proposed that an oxidation catalyst is enhanced in its oxidation potential by the presence of Mn.sub.3 O.sub.4 on the catalyst due to the combustion of MMT. Mn.sub.3 O.sub.4 is a weak oxidizing material by virtue of its ability to be reduced to MnO under rich conditions.
Automotive vehicle manufacturers have expressed concern with exhaust gas catalyst plugging--especially in light and medium duty truck engines. Early work showed the potential for catalyst plugging when vehicles were fueled with MMT gasoline (usually at the 1/8MMT level). In-depth studies of the plugging instances have related plugging to catalyst temperatures and flow variation, with high temperature, steady state operation having the greatest potential for catalyst plugging.
Manufacturers also have expressed concern about the impact of MMT plugging on efforts to meet light and medium duty truck emissions standards using oxidation catalysts. These catalysts have higher operating temperatures due to higher load operation and will be more subject to catalyst plugging problems than light duty vehicles. Also, foreign manufacturers have produced vehicles that operate at higher load and temperature than their domestic counterparts. These vehicles may be more susceptible to MMT catalyst plugging because of their smaller, highly loaded engines.
Additives in crankcase lubricating oils can also be a source of problems with the foregoing adverse effects of MMT-containing gasoline, as illustrated in a report in Automotive Engineering, November 1979, which reports on studies conducted by a large, U.S. automobile and truck manufacturer on both catalyst and oxygen sensor performance. The sensor is located in the exhaust stream ahead of the catalytic converter. Catalyst HC conversion efficiency at stoichiometric air/fuel (A/F) ratio and CO-NO.sub.x crossover efficiency decreased with increased amounts of zinc dialkyldithiophosphate (ZDP) in the lubricating oil and with increased phosphorus found on the catalyst. Alkaline metal additives in the oil reduced the amount of ZDP-derived phosphorus retained by the catalyst and reduced the deleterious effect of phosphorus on HC conversion efficiency, but had no effect on the reduction in CO-NOx crossover efficiency. Alkaline metal additives had no effect on sensor performance.
The sensor consisted of a zirconia element separating two platinum electrodes. The inner electrode is exposed to ambient air supplied through a hollow center terminal. The outer electrode is contacted by exhaust gases which pass through a louvered metal shield. At engine operating temperatures, the zirconia-platinum element acts as a galvanic cell.
The catalyst contains platinum (0.045 mass percent) and rhodium (0.019 mass percent) dispersed in a spherical shell near the surface of the alumina support pellets.
Since crankcase lubricating oils migrate, depending on engine wear conditions, into the combustion chamber, it is evident that sodium or barium compounds therein can be a source, in the presence of MMT in the gasoline, for the troublesome glass-like oxides of Na and Mn, Ba and Mn and/or Na, Ba and Mn, discussed above. Accordingly, it is desirable, if not essential, to exclude from crankcase lubricating oils, as well as from gasoline, organic and inorganic compounds of sodium and barium.